In recent years, studies on optical information recording methods have been carried out in various places in a field of information recording. The optical information recording methods have a number of advantages such that noncontact recording and reproducing can be performed and adopting to memory forms of read-only type, write-once-read-many type and rewritable type is possible, and a wide use thereof from industrial use to consumer use has been considered as methods of realizing an inexpensive large capacity file.
Achieving of a large capacity of optical recording media (hereinafter, also referred to as optical disks) for the above variety of optical information recording methods has been made mainly by shortening the wavelength of the laser light used as a light source in an optical information recording method and by increasing the numerical aperture (NA) of the objective lens to make a spot size small on a focal plane.
For example, in CDs (compact disks), a laser light wavelength is 780 nm, a numerical aperture (NA) of an objective lens is 0.45 and a capacity is 650 MB, while in DVD-ROMs(digital versatile disk—read only memory), the laser light wavelength is 650 nm, the NA is 0.6 and the capacity is 4.7 GB.
Furthermore, in an optical disk system of the next generation, a large capacity of 22 GB or more can be attained by making the laser light wavelength 450 nm or less and the NA 0.78 or more by using an optical disk wherein a thin light transmitting protective film (a cover layer) of, for example, 100 μm or so is formed on an optical recording layer.
FIG. 1A is a schematic perspective view of an optical disk for the above optical disk system of the next generation.
An optical disk D has an approximately disk shape with a center hole CH formed at its center portion and is driven to rotate in the drive direction DR.
When recording or reproducing information, a laser light of a blue to bluish violet color region or other light LT is irradiated on an optical recording layer in the optical disk DC by an objective lens OL having a numerical aperture of, for example, 0.8 or more for being used.
FIG. 1B is a schematic cross-sectional view of the optical disk shown in FIG. 1A, and FIG. 1C is an enlarged cross-sectional view of a key portion of the schematic cross-sectional view of FIG. 1B.
One surface of a disk substrate 13 made by a polycarbonate resin, etc. having a thickness of 1.1 mm or so is provided with grooves 13a for separating track regions and formed with an optical recording layer 14 comprising for example a reflection film, a dielectric film, a recording film, another dielectric film, etc. stacked in this order. The configuration and the number of layers of the optical recording layer 14 differ in accordance with the type of recording material and design.
The above recording film is, for example, a recording film of a phase-change type, a magneto-optical recording film or a recording film including an organic dye.
Furthermore, a light transmitting protective film 37 having a film thickness of 0.1 mm composed of an adhesive layer 35 made by an adhesive or a pressure-sensitive tackiness agent film, etc. and a polymer film 36 is formed on the optical recording layer 14.
When recording or reproducing information on or from the above optical disk, a light LT, such as a laser light, is irradiated on the optical recording layer 14 by the objective lens OL from the protective film 37 side.
When reproducing information from the optical disk, a returned light reflected on the optical recording layer 14 is received by a light receiving element, a predetermined signal is generated by a signal processing circuit and a reproduction signal is taken out.
In the optical disk as above, the optical recording layer 14 has irregularity shapes in accordance with the grooves 13a provided on one surface of the disk substrate 13, and the track regions are separated by the grooves 13a. 
For example, the regions which are projecting to the protective film 37 side when viewing from the disk substrate 13 side are referred as “lands” L, while recessed regions are referred as “grooves” G.
For example, a land/groove recording method for recording information both on the land and the groove can be adopted, and also only one of the land and groove may be used as a recording area.
The above optical disk is an optical disk of a type of irradiating a laser light to the optical recording layer 14 through the protective film 37 and reading a reflected light, and thereby, it can deal with an increased numerical aperture.
A method of producing the above optical disk shown in FIG. 1A to FIG. 1C will be explained.
First, a disk substrate 13 made by a polycarbonate resin, etc. having a film thickness of 1.1 mm or so being formed on its surface an irregularity pattern for an optical recording layer is formed, for example, by injection molding, on which an optical recording layer 14 having a stacked layer body of a reflection film, a dielectric film, a recording film, another dielectric film is formed on the irregularity pattern in this film forming order, for example, by a spattering method, so that the optical recording layer 14 having a pattern corresponding to the above irregularity pattern is formed.
Next, a polymer film 36 is adhered on the optical recording layer 14 by an adhesive 35, such as an ultra-violet curing resin adhesive or a pressure-sensitive tackiness agent film, so as to obtain a protective film 37 composed of the adhesive layer 35 and the polymer film 36.
From the above, an optical disk having the configuration shown in FIG. 1 can be obtained.
In the optical disk of the above configuration, however, since the polymer film shrunk during a long time use, there was a problem that warps arose on the optical disk.
An amount of a coma aberration generated when the disk warp above (tangential or radial skew) arises is proportional to NA3/λ, so when shortening wavelength of the laser light and increasing numerical aperture for a larger capacity, properties required to the optical disk become strict to suppress the coma aberration and the disk warp has to be suppressed.
The disk warp above is described to be in the radial direction and in the tangential direction, but due to shrinkage of the polymer film, a warp mainly in the radial direction is largely changed.